C-00001-字符处理

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define MAX_COLS 20
#define MAX_INPUT 1000

int read_column_numbers(int columns[],int max);
void rearrange(char *output,char const *input,int n_columns,int const columns[]);

int main(void)
{
int n_columns;
int columns[MAX_COLS];
char input[MAX_INPUT];
char output[MAX_INPUT];

n_columns = read_column_numbers(columns,MAX_COLS);

while(gets(input) != NULL)
{
printf("Original input:%s\n",input);
rearrange(output,input,n_columns,columns);
printf("Rearranged line:%s\n",output);
}
return EXIT_SUCCESS;
}
int read_column_numbers(int columns[],int max)
{
int num = 0;
int ch;
while(num<max && scanf("%d",&columns[num]) ==1 && columns[num] >= 0)
num += 1;
if(num % 2 != 0)
{
puts("Last column number is not paired.");
exit(EXIT_FAILURE);
}
while((ch = getchar())!=EOF && ch != '\n')
;
}
void rearrange(char *output,char const *input,int n_columns,int const columns[])
{
int col;
int output_col;
int len;

len = strlen(input);
output_col = 0;

for(col = 0; col < n_columns ;col += 2)
{
int nchars = columns[col+1] - columns[col] + 1;
if(columns[col] >= len || output_col == MAX_INPUT -1)
break;
if(output_col + nchars > MAX_INPUT -1)
nchars = MAX_INPUT - output_col -1;
strncpy(output + output_col,input + columns[col],nchars);
output_col += nchars;
}
output[output_col] = '\0';
}